When fabricating integrated circuits, layers of insulating, conducting and semiconducting materials are deposited and patterned to produce desired structures. “Back end” or metallization processes include contact formation and metal line or wire formation. Contact formation vertically connects conductive layers through an insulating layer. Conventionally, contact vias or openings are formed in the insulating layer, which typically comprises a form of oxide such as borophosphosilicate glass (BPSG) or oxides formed from tetraethylorthosilicate (TEOS) precursors. The vias are then filled with conductive material, thereby interconnecting electrical devices and wiring above and below the insulating layers. The layers interconnected by vertical contacts typically include horizontal metal lines running across the integrated circuit. Such lines are conventionally formed by depositing a metal layer over the insulating layer, masking the metal layer in a desired wiring pattern, and etching away metal between the desired wires or conductive lines.
Damascene processing involves forming trenches in the pattern of the desired lines, filling the trenches with a metal or other conductive material, and then etching the metal back to the insulating layer. Wires are thus left within the trenches, isolated from one another in the desired pattern. The etch back process thus avoids more difficult photolithographic mask and etching processes of conventional metal line definition.
In an extension of damascene processing, a process known as dual damascene involves forming two insulating layers, typically separated by an etch stop material, and forming trenches in the upper insulating layer, as described above for damascene processing. After the trenches have been etched, a further mask is employed to etch contact vias downwardly through the floor of the trenches and the lower insulating layer to expose lower conductive elements where contacts are desired.
Conductive elements, such as gate electrodes, capacitors, contacts, runners and wiring layers, must each be electrically isolated from one another for proper integrated circuit operation. In addition to providing insulating layers around such conductive elements, care must be taken to prevent diffusion and spiking of conductive materials through the insulating layers, which can cause undesired short circuits between among devices and lines. Protective barriers are often formed between via or trench walls and metals in a substrate assembly, to aid in confining deposited material within the via or trench walls. Barriers are thus useful for damascene and dual damascene interconnect applications, particularly for small, fast-diffusing elements such as copper.
Candidate materials for protective barriers should foremost exhibit effective diffusion barrier properties. Additionally, the materials should demonstrate good adhesion with adjacent materials (e.g., oxide via walls, adhesion layers, etch stop layers and/or metallic materials that fill the vias and trenches). For many applications, a barrier layer is positioned in a current flow path and so must be conductive. Typically, barriers have been formed of metal nitrides (MNx), such as titanium nitride (TiN), tantalum nitride (TaN), and tungsten nitride (WN), which are dense and adequately conductive for lining contact vias, wiring trenches, and other conductive barrier applications.
These lined vias or trenches are then filled with metal by any of a variety of processes, including chemical vapor deposition (CVD), physical vapor deposition (PVD), and electroplating. For effective conductivity and to avoid electromigration during operation, the metal of a contact or wiring layer should fill the via or trench without leaving voids or key holes. Completely filling deep, narrow openings with conductive material is becoming ever more challenging as integrated circuit dimensions are constantly scaled down in pursuit of faster operational processing speeds and lower power consumption.
As illustrated in FIGS. 1 to 2, utilizing a conductive barrier layer and/or other liners makes filling the trenches and vias of dual damascene processing even more difficult. FIG. 1 illustrates a dual damascene process in which an upper insulating layer 10 is formed over a lower insulating layer 12, which is in turn formed over a conductive wiring layer 14, preferably with an intervening dielectric diffusion barrier 15. This dielectric barrier 15 serves to prevent copper or other conductive material of the underlying runner 14 from diffusing into the overlying dielectric layer 12.
A mask is employed to pattern and etch trenches 16 in a desired wiring pattern. In the illustrated embodiment, the trench 16 is etched down to the level of an etch stop layer 19, which is formed between the two insulating layers 10, 12. This etch stop layer 19 is typically patterned and etched, prior to deposition of the upper insulating layer 10, to form a hard mask that defines horizontal dimensions of desired contact vias that are to extend from the bottom of the trench 16. Continued etching through the hard mask 19 opens a contact via 20 from the bottom of the trench 16 to the lower conductive wiring layer 14. FIG. 1 also shows an upper etch stop or chemical mechanical polishing (CMP) stop layer 21 over the upper insulating layer 10 to stop a later planarization step, as will be appreciated by the skilled artisan.
Protective liners 22, preferably formed of conductive material, are then formed on the exposed horizontal and sidewall surfaces. Typically, the liners 22 at least include a metal nitride, and may additionally include adhesion enhancing and seeding layers. For example, the liner 22 can comprise a tri-layer of Ti/TiN/Cu. In such a structure, the titanium layer serves to improve adhesion with exposed oxide sidewalls; the titanium nitride serves as a diffusion barrier; and a thin copper layer serves as a seed for later electroplating of copper. In other examples, the liners 22 can include tantalum nitride or tungsten nitride barriers.
Conformal deposition of the liners 22, however, is very difficult with conventional processing. For example, physical vapor deposition (PVD), such as sputtering, of a metal layer (for adhesion, barrier and/or seed layer) requires at least about 50 Å over all surfaces of the trench 16 and contact via 20. Unfortunately, PVD of metal into high aspect ratio voids necessitates much greater deposition on the top surfaces of the workpiece to produce adequate coverage of the via bottom. For example, typical state-of-the-art trench and contact structures for dual damascene schemes require about 500 Å PVD metal in order for 50 Å of metal to reach the bottom and sidewalls of the contact 20.
This poor step coverage is a result of the high aspect ratio of voids formed for dual damascene processing in today's integrated circuit designs. The aspect ratio of a contact via is defined as the ratio of depth or height to width. In the case of dual damascene contacts, the trench 16 and contact via 20 together reach through two levels of insulating layers 10, 12, such that the effective aspect ratio of the via 20 is very high.
Conventional deposition processes produce very poor step coverage (i.e., the ratio of sidewall coverage to field or horizontal surface coverage) of such high aspect ratio vias for a variety of reasons. Due to the directionality of PVD techniques, for example, deposition tends to accumulate more rapidly at upper corners 26 of the trench 16 and upper corners 28 of the via 20, as compared to the via bottom 30. As a result of the rapid build-up of deposited material the upper surfaces of the structure, the liners occupy much of the conductive line width in the trench 16 and even more, proportionately, of the contact via 20. These built-up corners 26, 28 then cast a shadow into the lower reaches of the structure, such that lower surfaces, and particularly lower corners, are sheltered from further deposition. Although PVD deposition can be directed more specifically to the via bottom, e.g., by collimation or by ionization of the depositing vapor, such additional directionality tends to sacrifice sidewall coverage.
Chemical vapor deposition (CVD) processes have been developed for certain metals and metal nitrides. CVD tends to exhibit better step coverage than PVD processes. In order for CVD processes to exhibit good step coverage, the reaction must be operated in the so-called “surface controlled” regime. In this regime, reaction species do not adhere to trench or via walls upon initial impingement. Rather, the species bounce off trench/via surfaces several times (e.g., 10–500 times) before reacting.
State-of-the-art CVD processes for depositing barrier layers at temperatures sufficiently low to be compatible with surrounding materials do not operate completely within the surface-controlled regime. Accordingly, even CVD processes, tend to deposit far less material at the bottom of a dual damascene contact 20 then on the upper surfaces and sidewalls of the structure. The upper corners of the trench 16 and the contact 20 represent a high concentration of surface area to volume. Deposition upon the horizontal upper surfaces and adjacent vertical sidewall surfaces merge together to result in an increased deposition rate near the corners 26, 28. Additionally, flowing reactants diffuse slowly into the confined spaces of the trench 16 and contact 20. Accordingly, the concentration of reactants reaching the via bottom 30 is far reduced relative to the concentration of reactants reaching upper surfaces of the structure. Thus, while somewhat improved relative to PVD, CVD step coverage of dual damascene structures remains uneven with most currently known low temperature CVD techniques.
In the pursuit of faster operational speeds and lower power consumption, dimensions within integrated circuits are constantly being scaled down. With continued scaling, the aspect ratio of contacts and trenches continues to increase. This is due to the fact that, while the width or horizontal dimensions of structures in integrated circuits continues to shrink, the thickness of insulating layers separating metal layers cannot be commensurately reduced. Reduction of the thickness in the insulating layers is limited by the phenomenon of parasitic capacitance, whereby charged carriers are slowed down or tied up by capacitance across dielectric layers sandwiched by conductive wires. As is known, such parasitic capacitance would become disabling if the insulating layer were made proportionately thinner as horizontal dimensions are scaled down.
With reference to FIG. 2, a scaled-down version of FIG. 1 is depicted, wherein like parts are referenced by like numerals with the addition of the suffix “a.” As shown, continued scaling leads to a more pronounced effect of uneven step coverage while lining dual damascene structures. Material build-up at the corners 28a of the contact via 20a quickly reduces the size of the opening, even further reducing the concentration of reactants that reach into the contact via 20a. Accordingly, coverage of the via bottom surface 30a drops off even faster. Moreover, the percentage of the trench 16a occupied by the liner materials is much greater for the scaled down structure of FIG. 2. Since the lining material is typically less conductive than the subsequent filler metal (e.g., copper), overall conductivity is reduced. Worse yet, cusps at the corners 28a of the contact via can pinch off before the bottom 30a is sufficiently covered, or during deposition of the filler metal.
Accordingly, a need exists for more effective methods of lining trenches and vias in integrated circuits, particularly in the context of dual damascene metallization.